In Rel-9, the Universal Mobile Telecommunications System (UMTS) standard introduced Dual-Carrier HSUPA (DC-HSUPA), which aims to increase the uplink data throughput by allowing a wireless device to transmit on two uplink carriers at the same time. In Rel-13, Dual-band Dual-Carrier HSUPA (DB-DC HSUPA) was added to the standard. DB-DC HSUPA aims at configuring two uplink carriers on different frequency bands. However, when DB-DC HSUPA is configured, high-frequency carriers have smaller coverage relative to low-frequency carriers. For example, there is a coverage difference of approximately 7.3 dB between a carrier operating at 900 MHz and a carrier operating at 2.1 GHz.
Until recently, the UMTS standard allowed configuration of a Transmission Time Interval (TTI) equal to 2 ms or 10 ms in the case of single carrier HSUPA. However, the UMTS standard only allows for configuring a TTI equal to 2 ms on both carriers for DB-DC HSUPA and/or DC-HSUPA. Nonetheless, and continuing with the evolution of the UMTS standard, a recently approved Rel-14 Work Item entitled “Multicarrier Enhancements for UMTS” will open the possibility of configuring 10 ms TTI on one or both uplink carrier frequencies in DB-DC HSUPA and/or DC-HSUPA scenarios.
Enabling different TTI configuration per carrier in DB-DC HSUPA and/or DC-HSUPA opens the possibility of having a variety of scenarios. For example, since 10 ms TTI offers a better coverage than 2 ms TTI, then in DB-DC HSUPA 10 ms TTI may be configured on a high frequency band and 2 ms TTI on the low frequency band. Such a configuration may provide compensation, even if slight, for the different propagation properties associated with each band. At a given path loss ratio, the compensation given by the TTI, if any, may be only approximately 1 dB or 2 dB, while the coverage difference between 900 MHz band and 2.1 GHz band may be approximately 7.3 dB.
The mixed TTI configuration for Uplink (UL) Multicarrier may have some side effects on some procedures and/or functionalities that currently only account for uplink transmissions to be performed on a 2 ms TTI basis. For example, prior to Rel-14, the standard stated that the power allocation in UL Multicarrier must be performed at the TTI boundary. As used herein, the term TTI boundary refers to the start of a TTI. However, it may be recognized that, upon the completion of Rel-14, the uplink carrier frequencies may be configured with different TTI boundaries. In that case, there will be two different TTI boundaries making power allocation problematic in those scenarios.
The Rel-14 “Multicarrier enhancements for UMTS” may also require a revision of some existing procedures and/or functionalities.